Chlorine is one of the most widely used bleaching agents in the bleaching of cellulosic fibrous material pulp, particularly in the bleaching of kraft pulp in the production of paper and paper products. However chlorine is coming under increasing attack because of the possible adverse affects on the environment, such as the production of effluents that are toxic to fish. Also, bleaching with chlorine alone means a fairly low ceiling of brightness.
It is well known that ozone is environmentally superior to chlorine in the bleaching of pulp, since ozone does not produce chlorinated ring compounds when it is used for bleaching pulp. However ozone is rarely practical in commercial installations because it is typically diluted in oxygen below a 10% concentration. Reactors for ozone therefore must deal with excess gas volume, or if ozone is utilized to ozonate a very dilute suspension of pulp, a large water volume must be dealt with. Also, ozone above a concentration of about 40% is violently explosive, and thus ozone must be handled carefully. This puts a practical limit on the concentration of ozone that may be utilized. Even at 40% concentration, there is too much ozone that is insoluble with oxygen to allow it to be used in conventional pulp processing.
According to the present invention it has been unexpectedly found that in the chlorination of pulp during bleaching to produce kraft pulp, the substitution of ozone for a part of the chlorine significantly reduces the adverse environmental consequences of the effluent, while allowing delignification to a greater extent than is possible with chlorine alone. The desired effects are achieved when ozone is added to a chlorine stream, that is when the chlorine and ozone are intimately mixed together and applied simultaneously to the pulp.
Practicing the invention it may also be possible to eliminate one or two stages from conventional bleach sequences in bleaching brown stock pulp to produce bleached kraft pulp. Typically four or five stages are necessary, however according to the invention by practicing an O.sub.3 /Cl.sub.2 stage, then an E.sub.o stage, and then a D stage, utilizing only three stages it should be possible to produce pulp having a brightness of 90 TAPPI Absolute or greater.
According to one aspect of the present invention there is provided: a method of bleaching cellulosic fibrous material pulp during the production of kraft pulp comprising the step of supplying a mixture of ozone and chlorine to the pulp to effect bleaching, the ozone and chlorine being applied simultaneously to the pulp, and the mixture of ozone and chlorine containing about 1% to the violent explosion limit of ozone (e.g. about 40%).
According to another aspect of the invention there is provided: a method of chlorine bleaching cellulosic fibrous material pulp during the production of kraft pulp without significantly affecting the degree of delignification produced by chlorine bleaching, comprising the step of: in the chlorine bleaching of the pulp, substituting ozone for a portion of the chlorine.
The invention also contemplates a method of producing 90 TAPPI Absolute, or greater, brightness kraft pulp by bleaching brown stock pulp in only three bleaching stages, the three bleaching stages comprising an O.sub.3 Cl.sub.2 stage in which ozone and chlorine are simultaneously applied to the pulp, an E.sub.o stage, and a D stage.
According to the invention a low fish toxicity effluent discharge from a kraft pulp bleaching facility produced by subjecting brown stock pulp to bleaching action of a mixture of about 1% to the violent explosion limit of ozone with chlorine, the ozone and chlorine being intimately simultaneously applied to the pulp to produce bleached pulp and effluent. The effluent has reduced total chlorinated ring compounds compared to obtaining comparable bleaching action utilizing chlorine alone.
It is the primary object of the present invention to provide for more effective and/or environmentally acceptable bleaching of pulp with chlorine during the production of kraft pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.